Exciton polariton optical switch

ABSTRACT

A waveguide having a periodic structure and the excitation of exciton polariton as one form of interactions between radiation fields and matter systems are applied to light switching to provide an optical switch that is excellent in light intensity extinction ratio and operable at a speed in the terahertz order. The optical switch includes a polariton and photon interacting region ( 5 ) made of a grating ( 3 ) formed on a top face of a transparent substrate ( 2 ) and a semiconductor layer ( 4 ) with which the grating ( 3 ) is covered. The polariton and photon interacting region ( 5 ) is irradiated from a free space with a controllable light ( 7 ) having a preestablished wavelength and also from a free space with a control light ( 6 ) having a preselected wavelength to control the transmissivity of the controllable light ( 7 ) through the polariton and photon interacting region ( 5 ).

TECHNICAL FIELD

The present invention relates to an optical switch that utilizes awaveguide having a periodic structure, and narrow band absorption linesby exciton polariton.

BACKGROUND ART

In digital light communications, demands for the higher informationtransmission rate, i.e., the higher bit rate know no limits. In theconventional optical switch, the light emission source may be a laserdiode whose current or voltage is controlled to turn its light emissionon and off, thereby forming a “1” and a “0” digital light signal. Such amethod of producing digital light signals by controlling the currentpassed through or the voltage applied to a laser diode, however, has itslimit in decreasing the parasitic circuit capacitance and inductance andincreasing the electron transit velocity, and hence has its limit inincreasing the bit rate.

Attempts have also be made to rapidly produce digital light signals withthe aid of a light modulator using an optical crystal such as LiNbO₃(lithium niobate) with a laser light incident thereon to give rise to anelectrooptic effect. This method has its limits, too, namely in matchingthe propagation speed of a light wave used and the propagation speed ofa microwave applied for controlling the light wave, and the bit rateachievable there must be at most in the GHz (giga-hertz) order.Moreover, with the microwave propagation loss that cannot be ignored,the method has the problem that the 0 by 1 ratio in light intensity,namely the light intensity extinction ratio remains unsatisfactory.

Amid the state of the art, with the progress in crystal growth andsemiconductor ultra-fine processing technologies made in recent years ithas become possible to make a semiconductor quantum structurepractically at will. This has also been followed by vigorous studies onthe interaction between radiation field and elementary excitation ofmaterials, using a photonic crystal or a semiconductor micro-resonator.Not only have the results of these studies contributed to theunderstanding of the fundamental physics, but also they are foundapplicable to the development of a high-performance optical device.

With the foregoing technological background, the present invention isaimed to provide an optical switch that has an excellent light intensityextinction ratio and is operable at a rate in the THz (tera-hertz)order, by applying to the switching of a light, a waveguide having aperiodic structure and the excitation of exciton polariton as one formof the interaction between a radiation field and an elementaryexcitation of a material.

DISCLOSURE OF THE INVENTION

In order to achieve the object mentioned above, there is provided inaccordance with the present invention an optical switch characterized inthat it comprises: a polariton and photon interacting region made of agrating formed on a top face of a transparent substrate and asemiconductor layer with which the said grating is covered; acontrollable light emitted from a free space and having a selectedwavelength and with which the said polariton and photon interactingregion is irradiated; and a control light for controlling transmissivityof the said controllable light through the said polariton and photoninteracting region.

The said control light preferably has a wavelength that brings about anoptical Stark effect of exciton without entailing an actual excitationof the said semiconductor layer.

The said grating is preferably formed so that its grating periodcorresponds to a length of m/2 of the wavelength of the saidcontrollable light in the said semiconductor layer where m is a positiveinteger.

Further, the said semiconductor layer may be layered in a groove of thesaid grating to a predetermined depth.

The said semiconductor layer is preferably a semiconductor layer that islarge in both exciton oscillator strength and exciton binding energy.The said semiconductor layer that is large in both exciton oscillatorstrength and exciton binding energy is preferably a semiconductor layerhaving a multiple quantum well structure made of units each of whichcomprises a pair of semiconductor quantum well and a barrier layer smallin dielectric constant and separating the said semiconductor quantumwells from each other.

The said multiple quantum well structure that is large in both excitonoscillator strength and exciton binding energy is specifically of alaminar or layered inorganic-organic perovskite semiconductor that isexpressed by chemical formula: (C_(n)H_(2n+1)NH₃)₂MX₄ where M=Pb or Snand X=I, Br or Cl and n is a positive integer.

The said polariton and photon interacting region is preferably formed onits top face with a highly refractive transparent material for lightconfinement. The said highly refractive transparent material for lightconfinement is preferably a polymer.

In the optical switch of the present invention constructed as mentionedabove, making the controllable light incident on the polariton andphoton interacting region perpendicularly thereto causes a standing waveof the controllable light to be formed in a direction of the gratingperiod of the grating. Photon of this standing wave is strongly coupledto exciton of the semiconductor to form exciton polariton and is therebyabsorbed. Thus, the controllable light is prevented from passing throughthe polariton and photon interacting region.

On the other hand, making both the controllable light and the controllight simultaneously incident on the polariton and photon interactingregion allows the energy of exciton in the semiconductor layer to berapidly changed by the control light causing the optical Stark effect tochange the dispersion relation of polariton that is a state that excitonand photon are strongly coupled, thereby changing the photon energy ofthe standing wave strongly coupled with exciton of the semiconductor.Thus, the controllable light is prevented from coupling with exciton andis thereby allowed to pass through the polariton and photon interactingregion. Selecting the grating period allows the wavelength of the lightforming the standing wave to be selected.

Also, increasing the number of the grooves forming the grating allowssetting the very narrow half-width of the wavelength of the light thatforms the standing wave. Moreover, the control means for controlling thetransmissivity by bringing about the third order optical nonlineareffect, preferably the optical Stark effect, permits controlling thecontrollable light so as to render it transmissible and nontransmissiblevery rapidly or at an ultra-high speed.

Further, the use of a semiconductor layer that is large in both excitonoscillator strength and exciton binding energy allows almost everyphoton of the standing wave to be converted to exciton polariton,thereby further raising the light intensity extinction ratio of theoptical switch.

If the semiconductor layer that is large in both exciton oscillatorstrength and exciton binding energy is constituted by a semiconductorlayer having a multiple quantum well structure made of units each ofwhich comprises a pair of semiconductor quantum well and a barrier layersmall in dielectric constant and separating the said semiconductorquantum wells from each other, then extremely high exciton oscillatorstrength and exciton binding energy are obtained.

Further, the use of a laminar or layered inorganic-organic perovskitesemiconductor that is expressed by the chemical formula:(C_(n)H_(2n+1)NH₃)₂MX₄ where M=Pb or Sn and X=I, Br or Cl and n is apositive integer, provides for a multiple quantum well structure that islarge in both exciton oscillator strength and exciton binding energy.

Further, if the polariton and photon interacting region is formed on itstop face with a highly refractive transparent material for lightconfinement, then the improvement in the light confinement into thepolariton and photon interacting region still further enhances the lightintensity extinction ratio of the optical switch.

Constructed as mentioned above, the optical switch according to thepresent invention is operable extremely rapidly, namely at a speed inthe tera-hertz order and is excellent in light intensity extinctionratio.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will better be understood from the followingdetailed description and the drawings attached hereto showing certainillustrative embodiments of the present invention. In this connection,it should be noted that such forms of embodiment illustrated in theaccompanying drawings hereof are intended in no way to limit the presentinvention but to facilitate an explanation and understanding thereof. Inthe drawings:

FIGS. 1A and 1B are typical views illustrating a makeup and anoperation, respectively, of an exciton polariton optical switchaccording to the present invention, wherein FIG. 1A shows the of theexciton polariton optical switch and FIG. 1B is a graph showing how thedip of the intensity of a light for transmission changes when a controllight is switched on and off;

FIG. 2 is a diagram illustrating how the controllable light is dispersedin a grating;

FIG. 3 is a diagram illustrating how the exciton polariton is dispersed;

FIGS. 4A and 4B are schematic diagrams illustrating a structure ofC_(n−)MX₄ used to form a semiconductor layer of an exciton polaritonoptical switch of the present invention where FIG. 4A shows astereostructure of a unit cell (with the alkyl chain omitted) and FIG.4B is a schematic projection diagram of its crystallographic structurein the directions of a- and b-axes;

FIG. 5 is a diagram illustrating the distribution of electric flux linesof exciton in a quantum well layer in the semiconductor layer of anexciton polariton optical switch of the present invention;

FIG. 6 is a table showing actual measurements of exciton binding energyE_(b), band gap E_(g), exciton absorbing energy E_(ex), excitonoscillator intensity f_(ex), quantum well width L_(well), quantum welldielectric constant ε_(w) and barrier layer dielectric constant ε_(b) ofPhE-PbI4 used in a specific implementation of the present invention;

FIG. 7 is a typical cross sectional view illustrating the makeup of anexciton polariton optical switch used in the specific implementation;

FIG. 8 is a diagram illustrating a measuring system used for themeasurement of transmission spectra in the specific implementation;

FIG. 9 is a diagram illustrating the transmission spectra of the excitonpolariton optical switch in the specific implementation;

FIG. 10 is a schematic diagram illustrating a measuring system used forthe measurement of the working speed of the exciton polariton opticalswitch in the specific implementation;

FIG. 11 is a diagram illustrating changes of transmissivity of theexciton polariton optical switch with time in the specificimplementation wherein (A) shows its linear transmissivity plotted alongthe left hand side ordinate axis and the spectrum of a control light interms of the light intensity plotted along the right hand side ordinateaxis, both with respect to the wavelength plotted along the abscissaaxis and (B) shows differential spectra which were measured out with thedelay time shifted approximately every 53 fs and which are plotted alongthe ordinate axis with respect to the wavelength plotted along theabscissa axis; and

FIG. 12 is a graph illustrating the dependency upon the delay time ofeach of the differential transmissions shown at the broken lines A and B(wave lengths: 533.5 nm and 536.7 nm) in the diagram of FIG. 11(B).

BEST MODES FOR CARRYING OUT THE INVENTION

An explanation is given in detail below in respect of forms ofembodiment of the present invention with reference to the accompanyingdrawing Figures wherein the same reference characters are used todesignate the substantially same members or components.

FIGS. 1A and 1B are typical views illustrating a makeup and anoperation, respectively, of an exciton polariton optical switchaccording to the present invention, wherein FIG. 1A shows the makeup ofthe exciton polariton optical switch and FIG. 1B is a graph showing howthe dip of the transmission intensity of a controllable light changeswhen a control light is switched on and off

Referring to FIG. 1A, an exciton polariton optical switch 1 according tothe present invention comprises a transparent substrate 2 formed on itstop face with a grating 3 having a grating period Λ, and a semiconductorlayer 4 with which the grating 3 is covered to coat the same. The regionof the grating 3 covered with the semiconductor layer 4 and designatedby reference character 5 is herein referred to as “polariton and photoninteracting region”.

The exciton polariton optical switch 1 of the present invention can beoperated by externally applying thereto a control (controlling) light 6of a particular wavelength and a controllable (controlled) light 7 of aspecified wavelength.

FIG. 1B is a graph that shows how the light transmission of thepolariton and photon interacting region 5 is dependent upon photonenergy when it is irradiated with the control light 6 and when it isnot, respectively. In the graph of FIG. 1B, the photon energy is plottedalong the abscissa axis and the light transmission is plotted along theordinate axis. The solid and broken lines indicate the dependencies onthe photon energy of the light transmissions of the polariton and photoninteracting region 5 when it is irradiated with the control light 6 andwhen it is not, respectively. There are shown dips 7 a and 7 b which aredips by absorption of exciton polariton in the presence and absence ofthe control light, respectively; and dips 8 a and 8 b which are dips byabsorption of exciton of the semiconductor layer 4 in the presence andabsence of the control light 6, respectively.

For operating the exciton polariton optical switch 1 of the presentinvention, the photon energy of the controlled light 7 is madecoincidence with the the exciton polariton absorption dip 7 b in theabsence of the control light 6. In the absence of the control light 6,the controlled light 7 is absorbed by the exciton polariton absorptiondip 7 b and is not transmitted through the polariton and photoninteracting region 5. Irradiating the polariton and photon interactingregion 5 with the control light 6 causes the exciton polaritonabsorption dip to be shifted to 7 a and allows the controlled light 7 tobe transmitted through the polariton and photon interacting region 5.

As opposed to the above, it is also possible to make the photon energyof the controlled light 7 to be coincidence with the exciton polaritonabsorption dip 7 a in the presence of the control light 6. Then, thecontrolled light 7 is absorbed by the exciton polariton absorption dip 7a and is not transmitted through the polariton and photon interactingregion 5 while it is being irradiated with the control light 6. Cuttingoff the control light 6 shifts the exciton polariton absorption dip to 7b, and allows the controlled light 7 not absorbed to pass through thepolariton and photon interacting region 5. In this manner, the excitonpolariton optical switch 1 of the present invention operates to make ittransparent and nontransparent to the controlled light 7 when thecontrol light 6 is turned on and off.

An explanation is next given in respect of the operating principles ofan exciton polariton optical switch according to the present invention.

At the outset, mention is made of how a controllable light is dispersedin a grating. FIG. 2 is a diagram illustrating how the controllablelight 7 is dispersed in the grating waveguide 3. In the graphs of FIG.2, the propagation vector k_(x) of space harmonics propagating throughthe grating waveguide 3 is plotted along the abscissa axis while thephoton energy E is plotted along the ordinate axis. The solid lines 10represent a simplified dispersion relation of modes of light wavesguided through the grating waveguide 3, and is expressed by equation:

E=hck _(x)/2πn*  (1)

where h is Planck's constant, c is the velocity of light and n* is theeffective refractive index of the waveguide. The broken lines 11indicate dispersion relations of space harmonics diffracted by areciprocal vector G_(m)(=2πm/Λ where m is an integer and Λ is thegrating period). The space harmonics are coupled together at the pointsof intersection of these straight lines to form a standing wave whoseenergy is given by combining together a condition in which it is formed,namely kx−(−kx)=G_(m), and the equation (1) above, as follows:

E=hcm/2n*Λ  (2)

Mention is next made of the interaction between the exciton polaritonand the standing wave existing in the polariton and photon interactingregion 5.

Exciton polariton is a strongly coupled state of photon and exciton in asemiconductor.

FIG. 3 shows how exciton polariton is dispersed. FIG. 3A is a diagramthat illustrates dispersion relations, indicated by the solid lines 12 aand 12 b, for exciton polariton in a waveguide without a grating and adispersion relation, indicated by the broken line 10, for a mode oflight wave guided through the waveguide. E₀ represents excitonabsorption energy. Exciton polariton, which is a strongly coupled stateof photon and exciton in a semiconductor, has two eigenstates, and thedispersion relations attributable to these respective eigenstates arereferred to as polariton's upper and lower branch modes (12 a) and (12b), respectively.

FIG. 3B is a diagram that illustrates dispersion relations, indicated bythe solid lines 12 a and 12 b, for exciton polariton in a waveguide witha grating, namely in the polariton and photon interacting region 5 hereand dispersion relations, indicated by the broken lines 11, for a modeof light wave guided through the waveguide with the grating. ΔErepresents dissociation energy, namely a difference between the energyof the standing wave given by the equation (2) and the excitonabsorption energy E₀, and is given as follows:

ΔE=hcm/2n*Λ−E ₀  (3)

If ΔE is negative, then the exciton polariton corresponding to the lowerbranch modes 12 b close in energy to the standing wave is excited andthe standing wave is thereby absorbed. Thus, the controlled light 7forming the standing wave in the polariton and photon interacting region5 acts to excite exciton polariton and is thereby absorbed.

Mention is next made of how the transmissivity is controlled by thecontrol light.

For controlling the position of the exciton polariton absorption dipwith the control light 6, use is made of a light of a wavelength capableof giving rise to a third order nonlinear optical effect. To wit,irradiating the semiconductor layer 4 with the control light 6 of such awavelength causes the energy of exciton to change rapidly and allows adispersion relation of exciton polariton that is a strongly coupledstate of exciton and photon, to so change, thereby permitting the photonenergy of the standing wave strongly coupled to exciton in thesemiconductor to so change. Since this phenomenon occurs in thefemto-second (fs), an exciton polariton optical switch of the presentinvention can be operated in a terahertz (THz) band.

Mention is next made of the optical intensity extinction ratio of anexciton polariton optical switch according to the present invention.

To wit, good optical intensity extinction ratio requires a highprobability at which the controlled light forming the standing wave inthe polariton and photon interacting region 5 is strongly coupled withexciton polariton.

This in turn requires the semiconductor layer 4 to be of a multiplequantum well structure, the exciton oscillator intensity f_(ex) to belarge, and the exciton binding energy E_(b) to be large. In the excitonpolariton optical switch of the present invention, use is made of thesemiconductor layer 4 having a multiple quantum well structure, a largeexciton oscillator strength f_(ex) and a large exciton binding energyE_(b).

The semiconductor layer 4 for use in the exciton polariton opticalswitch of the present invention may be made of a layered or laminarinorganic-organic perovskite semiconductor that is expressed by chemicalformula: (CnH_(2n+1)NH₃)₂MX₄ where M=Pb or Sn, X=I, Br or Cl and n is apositive integer, and is hereinafter referred to as C_(n−)MX₄.

FIGS. 4A and 4B are schematic diagrams illustrating structures ofC_(n−)MX₄ used to form the semiconductor layer 4 of an exciton polaritonoptical switch of the present invention wherein FIG. 4A shows astereostructure of a unit cell (with the alkyl chain omitted) and FIG.4B is a schematic projection diagram of its crystallographic structurein the directions of a- and b-axes.

The unit cell shown contains four (4) chemical units. [MX₆]⁴⁻ octahedron13 has an X⁻ ion positioned at each of its apexes and a M²⁺ ionpositioned at its center (and indicated by the black circle); they areionically bonded. Each of alkyl ammonium chains 14 has its NH₄ sitepositioned essentially at a center of four X⁻ ions as ionically bondedat the same height as the X⁻ ions, and extends externally from the[MX₆]⁴⁻ octahedral layer 13. And, an electrically neutral layer isformed of the layered alkyl ammonium chains 14 and coupled to the[MX₆]⁴⁻ octahedron 13 by a Van der Waals force.

The [MX₆]⁴⁻ octahedral layer 13 is a semiconductor of direct transitiontype, wherein if M is Pb, the uppermost valence band is made up of the6p orbital of Pb²⁺ hybridized by the 5p orbital of X⁻. For this reason,a large exciton oscillator intensity f_(ex) and hence strong excitonabsorption is exhibited. It is thus shown that the semiconductor layer 4used in the present invention is of a large exciton oscillator intensityf_(ex).

The alkyl ammonium chain layer 14 separating the [MX₆]⁴⁻ octahedrallayers 13 from one another is an insulator whose dielectric constant issmall. Accordingly, the alkyl ammonium chain layer 14 can function as abarrier layer for the [MX₆]⁴⁻ octahedral layers 13 so that the [MX₆]⁴⁻octahedral layers 13 and the alkyl ammonium chain layer 14 as theirbarrier layer together make up a quantum well. Such quantum wellslayered in the direction of C-axis provide for a multiple quantum wellstructure. It is thus shown that the semiconductor layer 4 for use inthe present invention is provided with a multiple quantum wellstructure.

It is further shown that the alkyl ammonium chain barrier layer 14having a small dielectric constant is large in exciton binding energy.

FIG. 5 shows the distribution of electric flux lines of exciton in aquantum well layer as mentioned above, in which e and h represent anelectron and a hole, respectively. The dielectric constant of the[MX₆]⁴⁻ octahedral layers 13 as the quantum wells and the dielectricconstant of the alkyl ammonium chain layers 14 as the barrier layers arerepresented by ε_(w) and ε_(b), respectively.

As will be apparent from the distribution of electric flux lines shownin FIG. 5, the electron e and the hole h which make up the exciton hereare effectively acted on by the Coulomb interaction through the alkylammonium chain layer 14 as the barrier layer that is small in dielectricconstant, and therefore provide for a large exciton binding energyE_(b).

In a specific implementation of the present invention, use is made ofM=Pb, X=I and the alkyl ammonium chain=(C₆H₅C₂H₄NH₃), namely of alayered or laminar perovskite semiconductor of the lead iodide family(hereinafter referred to as PhE-PbI4). FIG. 6 shows actual measurementsof exciton binding energy E_(b), band gap E_(g), exciton absorbingenergy E_(ex), exciton oscillator intensity f_(ex), quantum well widthL_(well), quantum well dielectric constant ε_(w) and barrier layerdielectric constant ε_(b) of PhE-PbI4 used.

It is thus shown that the exciton polariton optical switch according tothe present invention using the semiconductor layer 4 having a multiplequantum well structure and having large exciton binding energy E_(b),and large exciton oscillator intensity f_(ex) as seen from FIG. 6 ishigh in light intensity extinction ratio. Further, it is operable at aroom temperature.

Mention is next made in detail of this specific implementation of theinvention.

FIG. 7 is a view showing the makeup of the exciton polariton opticalswitch here implemented. The makeup of the exciton polariton opticalswitch shown in FIG. 7 differs from that shown in FIG. 1 in that thegrooves of the grating 3 are filled with semiconductor layers 4 up to acertain depth lower than their upper ends and that for the sake ofimproving their optical confinement, the top faces of thesesemiconductor layers are covered with a transparent dielectric layer 9of high refractivity, e.g., a polyethylene layer 9. In thisimplementation, the grating 3 is formed by etching the surface of aquartz substrate 2. The grating has a depth h of 0.3 μm, a line andspace ratio r of 1:4, and an area of 1.5 mm×1.5 mm. Five such gratingswere prepared having different grating periods Λ of 700 nm, 680 nm, 660nm, 640 nm and 620 nm, respectively. Use is made of PhE-PbI4 for thesemiconductor layers 4.

FIG. 8 is a diagram illustrating a measuring system used for themeasurement of transmission spectra in the specific implementation. Forthe light source, use is made of an iodine (halogen) lamp 15 as a whitelight source, whose output light 16 is condensed into a pin hole 17, ofwhich an output light 16 is polarized through a polarizing plate 18. Thedirection of polarization is made parallel to a groove in the grating 3of the exciton polariton optical switch 1. The polarized output light 16is condensed through an iris 19 on the obverse of the exciton polaritonoptical switch 1. The light transmitted through and out of the excitonand polariton optical switch 1 is condensed into a fiberopticalcondenser 20 and is measured by a spectrometer 21 to obtain itstransmission spectra.

FIG. 9 is a diagram illustrating the transmission spectra of the excitonpolariton optical switch in this specific implementation. In thediagram, the photon energy is plotted along the abscissa axis while thetransmission is plotted along the ordinate axis. Six specimens are usedfor the measurement, including those having the grating periods of 700nm, 680 nm, 660 nm, 640 nm and 620 nm and a comparative specimen that isdevoid of the grating.

As is seen from FIG. 9, an absorption dip 7 b appears in the side lowerin energy than where an exciton absorption dip 8 b appears, and isshifted more to the lower energy side as the grating period Λ becomesgreater, from which it is apparent that this absorption dip is anexciton polariton absorption dip.

Mention is next made of the working speed of the exciton polaritonoptical switch in this implementation.

FIG. 10 is a schematic diagram illustrating a measuring system used forthe measurement of the working speed of the exciton polariton opticalswitch in this specific implementation.

A laser pulse 31 of a wavelength of 760 nm and a time half-width of 150fs is generated by a laser light source 30 comprising an Er-doped fiberlaser and a titanium-sapphire regenerative amplifier and is split intotwo, with one of which a deuterium oxide cell 32 is irradiated toproduce a white light 33 and the other of which is converted by anoptical parametric oscillator 34 into a control light 6 having awavelength of 538 nm. The control light 6 has its delay time from thewhite light 33 adjusted by a delay time controller 35 that adjusts theoptical length of the control light 6 to adjust the delay time. Theexciton polariton optical switch 1 disposed in a thermostatic chamber 36is irradiated with the white light 33 and the control lights 6 delayedthereafter with various delay times, and the white light 33 transmittedthrough the exciton polariton optical switch 1 is condensed with afiberoptical condenser 37 and guided to a spectrometer 21 to measure outits transmission spectrum. Here, a portion of the white light is takenout with the fiberoptical condenser 37 to normalize the transmissionspectrum.

Each control light 6 has a central wavelength of 538 nm, a light energydensity of 30 μJ/cm², a time half-width of 200 fs and a repetition rateof 1 kHz. The control light 6 is made incident on the obverse of theexciton polariton optical switch 1 with an angle of inclination of 3degrees to a normal thereto. The polarization direction of the light ismade parallel to the grating grooves. The measurements were performed atthe room temperature. The differential transmission spectrum is aspectrum represented by a difference from the transmission spectrum inthe absence of the control light.

FIG. 11 is a diagram illustrating changes with time of transmissions ofthe exciton polariton optical switch in this specific implementationwherein (A) shows its linear transmissivity in the absence of thecontrol light plotted along the left hand side ordinate axis and thespectrum of a control light in terms of the light intensity plottedalong the right hand side ordinate axis, both with respect to thewavelength plotted along the abscissa axis and (B) shows differentialspectra which were measured out with the delay times shiftedapproximately every 53 fs and which are plotted along the ordinate axiswith respect to the wavelength plotted along the abscissa axis.

In FIG. 11(A), there are seen a wide dip 8 b broadened centering on awavelength of 520 nm, which is due to the semiconductor layer excitonabsorption, and sharp dips 7 b and 7 b′ at wavelengths of 535 nm and 550nm, respectively, which are the exciton polariton absorption dips. InFIG. 11(B), changes of the differential transmission spectra with thevarying delay time between the broken lines A and B in a wavelengthregion indicate the dip 7 b shown in FIG. 11(A) shifting successivelyunder the influence of irradiation with the control light 6.

As will be apparent from this diagram, with the lapse of time anincrease in the transmission occurs in the side longer in wavelengththan the center of the dip 7 b (center between the broken lines A and B)and a decrease in the transmission occurs in the side shorter inwavelength than the center of the dip 7 b, thus revealing that the dip 7b under the influence of irradiation with the control light 6 shiftstowards the higher energy side.

FIG. 12 is a graph illustrating the dependency upon the delay time ofeach of the differential transmissions shown at the broken lines A and B(wave lengths: 533.5 nm and 536.7 nm) in the diagram of FIG. 11(B). Inthe graph, the delay time is plotted along the abscissa axis and theintensity of the differential transmission is plotted along the ordinateaxis.

As will be apparent from this graph, the time half-width is about 200fs, which is the same as that of the control light 6.

From these results it is seen that the exciton polariton optical switchof the present invention is operable at a frequency of at least 5 THz orhigher, and from the peak value in this graph and its background valuesit is also seen that the optical switch is extremely high in lightintensity extinction ratio.

The phenomenon that the dip under the influence of control lightirradiation shifts at an extra-high speed is considered as follows: Ithas hitherto been known that irradiating with a light of energy lowerthan the exciton absorbing energy at a high intensity shifts the excitonabsorbing energy rapidly to the higher energy side. This phenomenon,called the “optical Stark effect”, has been understood to be due to adressed state that exciton is dressed with photon.

The phenomenon that irradiation with control light causes the dip tovery rapidly shift is considered to be due to the fact that the opticalStark effect causes the exciton energy to change very rapidly, which inturn changes the dispersion relation of the exciton polariton which is astate that exciton and photon are strongly coupled, thereby changing thedip energy.

INDUSTRIAL APPLICABILITY

As will have been appreciated from the foregoing description, theoptical switch according to the present invention is an excitonpolariton optical switch that is excellent in light intensity extinctionratio and that is operable at an ultra-high speed in the terahertzorder, and hence is extremely useful when used as an optical switch tobe working very rapidly.

What is claimed is:
 1. An exciton polariton optical switch,characterized in that it comprises: a polariton and photon interactingregion made of a grating formed on a top face of a transparent substrateand a semiconductor layer with which said grating is covered; acontrollable light emitted from a free space and having a selectedwavelength and with which said polariton and photon interacting regionis irradiated; and a control means for controlling transmissivity ofsaid controllable light through said polariton and photon interactingregion.
 2. An exciton polariton optical switch as set forth in claim 1,characterized in that said third order optical nonlinear effect is anoptical Stark effect of exciton.
 3. An exciton polariton optical switchas set forth in claim 1, characterized in that said grating is formed sothat its grating period corresponds to a length of m/2 of the wavelengthof said controllable light in said semiconductor layer where m is apositive integer.
 4. An exciton polariton optical switch as set forth inclaim 1, characterized in that said semiconductor layer is layered in agroove of said grating to a preestablished depth.
 5. An excitonpolariton optical switch as set forth in claim 1, characterized in thatsaid semiconductor layer is a semiconductor layer that is large in bothexciton oscillator strength and exciton binding energy.
 6. An excitonpolariton optical switch as set forth in claim 5, characterized in thatsaid semiconductor layer that is large in both exciton oscillatorstrength and exciton binding energy is a semiconductor layer having amultiple quantum well structure made of units each of which comprises apair of semiconductor quantum wells and a barrier layer small indielectric constant and separating said semiconductor quantum wells fromeach other.
 7. An exciton polariton optical switch as set forth in claim6, characterized in that said multiple quantum well structure that islarge in both exciton oscillator strength and exciton binding energy isof a laminar or layered inorganic-organic perovskite semiconductor thatis expressed by chemical formula: (C_(n)H_(2n+1)NH₃)₂MX₄ where M=Pb orSn and X=I, Br or Cl and n is a positive integer.
 8. An excitonpolariton optical switch as set forth in claim 1, characterized in thatsaid polariton and photon interacting region is formed on its top facewith a highly refractive transparent material for light confinement. 9.An exciton polariton optical switch as set forth in claim 8,characterized in that said highly refractive transparent material forlight confinement is a polymer.